It is a standard technique in magnetic recording to record multiple tracks of information onto a single magnetic medium. Such is the case, for example, with the left and right channels of a stereo recording. Although the information to be recorded in each of these channels is distinct, there is a common time relationship for the information contained in both of the channels which must be accurately maintained in order to ensure the information, in this case sound, is correctly interpreted or reproduced. This is often a problem since the transducer which is used to reproduce the information is ordinarily separate from the transducer which originally records the information onto the medium. Thus any variation in the alignment of the two transducers with respect to the medium will introduce a time difference, or phase error between the information as originally recorded and the information as reproduced. Even if the same transducer is used to reproduce the information as was used to record the information, misalignment can still occur due to a stretching of the medium or magnetic tape between the time of recording and reproducing, or a simple skewing of the tape with respect to the playback transducer during playback. This problem is especially manifest in the case of stereo broadcasting of the information contained on the medium, since a number of the listeners of stereo broadcasts utilize monophonic receivers. If phase errors occur between the left and right channels during the playback of a tape which is broadcast in stereo, it is possible that a listener utilizing a monophonic radio will hear a severely distorted signal. This is because the phase error between the left and right channels will cause an improper summation of these channels when they are mixed for monophonic reproduction. Tests conducted on current broadcast systems (i.e. those using cartridge type recorder/players) show that such systems find it difficult to consistently obtain a phase error of less than 45 degrees, and in fact, frequently produce a signal with a phase error as great as 360 degrees (as determined at the 12.5 kilohertz standard).
These values may sound extreme unless it is appreciated that the azimuth error between a head and the tracks on the medium need only be very minuscule for such a phase error to occur. For example, a 12 kHz tone requires 0.625 mil (15.9.times.10.sup.-4 cm) on a tape moving at 7.5 inches per second (19.1 cm/sec) to reproduce one cycle. If the various recorded tracks are longitudinally displaced by only 0.078 mil (1.98.times.10.sup.-4 cm) a resulting phase error of 45 degrees will be present in the reproduced signal. Assuming a 0.125 inch (0.3 cm) distance between the tracks, the azimuth error of the head which produces such a phase error is only 0.degree.2'.
Several manufacturers have attempted to design record/playback systems which contain mechanisms to rectify this phase error problem. One such manufacturer utilizes a master record machine which contains a servoed record head which is adjusted during a pre-record calibration step to null or eliminate any phase error of that record head with respect to that medium. This method is however cumbersome since it requires recalibration prior to every recording operation. This method also does not totally solve the problem since the prerecorded master tape is then played back on any one of several playback machines. Calibration of the master recorder, although correcting the phase error for the tape as it is recorded in that machine, does not prevent phase error from being introduced when that same tape is played back on a different machine. It is therefore highly desirable to correct the phase error during playback of the tape rather than during the recording.
Another manufacturer (see U.K. Patent GB No. 2,022,874A) scans the upper and lower halves during the playback of a given information track and determines the time difference between the zero cross-overs of the monitored signals. This time difference is then used to derive a control signal for compensating this monitored phase error. This method also is not without problems because of the presence of signals having varying frequencies and amplitudes. These signals require substantial filtering and processing before they can be used to measure any phase error.